Check how many records in a data file - sql-server

Question

I need to know how many records stored in single data file after carrying out table partitioning. Here I'm using Sales.SalesOrderDetail from AdventureWorks2016 sample database for demonstration.
I want TSQL query/script displaying the following output:
FileName Count
file1 27405
file2 56573
file3 37339
Here is the procedure I wrote:
Creating a database
CREATE DATABASE PTDB
Creating file group
ALTER DATABASE PTDB
ADD FILEGROUP MyFileGroup
Creating 4 files to store the split table records (I'm using X instead of repeating the code 4 times):
ALTER DATABASE PTDB
ADD FILE
(
NAME = 'fileX',
FILENAME = 'D:\SQLServer2017Media\MSSQLSERVER\MSSQL14.MSSQLSERVER\TablePt\ptX.ndf',
SIZE = 120,
FILEGROWTH = 10
)
TO FILEGROUP MyFileGroup
Creating partition function and partition scheme:
CREATE PARTITION FUNCTION fnOrderYears(DateTime)
AS RANGE LEFT FOR VALUES
('2012-12-31','2013-12-31','2014-12-31')
GO
CREATE PARTITION SCHEME psOrderYearsFiles
AS PARTITION fnOrderYears ALL TO (MyFileGroup)
GO
Creating the partitioned table and load the data from AdventureWorks2016.Sales.SalesOrderDetail table:
CREATE TABLE [dbo].[SalesOrderDetail] (
[SalesOrderID] [int] NOT NULL,
[SalesOrderDetailID] [int]NOT NULL,
CONSTRAINT SalesOrderDetailPK PRIMARY KEY
NONCLUSTERED(SalesOrderDetailID),
[CarrierTrackingNumber] [nvarchar](25) NULL,
[OrderQty] [smallint] NOT NULL,
[ProductID] [int] NOT NULL,
[SpecialOfferID] [int] NOT NULL,
[UnitPrice] [money] NOT NULL,
[UnitPriceDiscount] [money] NOT NULL,
[LineTotal] money,
[OrderDate] [datetime] NOT NULL
)
GO
INSERT INTO [dbo].[SalesOrderDetail]
(
SalesOrderID, SalesOrderDetailID, CarrierTrackingNumber,
OrderQty, ProductID, SpecialOfferID, UnitPrice, UnitPriceDiscount,
LineTotal, OrderDate
)
SELECT
SalesOrderID, SalesOrderDetailID, CarrierTrackingNumber,
OrderQty, ProductID, SpecialOfferID, UnitPrice, UnitPriceDiscount,
LineTotal, ModifiedDate
FROM AdventureWorks2016.Sales.SalesOrderDetail
BTW, I'm using this SQL query but I'm not sure if it is right:
SELECT $PARTITION.fnOrderYears(OrderDate) AS Partition,
COUNT(*) AS Count
FROM SalesOrderDetail
GROUP BY $PARTITION.fnOrderYears(OrderDate)
ORDER BY Partition;